With rapid development of mobile communications technologies, providing a device-to-device proximity service (D2D ProSe) on a mobile communications network has become a research topic for a long term evolution (LTE) system Rel. 12 of the third generation partnership project (3GPP). A D2D ProSe provided based on a physical layer of an LTE network greatly enriches a service scope of the LTE network. The D2D ProSe includes D2D discovery and D2D direct communication. D2D discovery refers to a process in which a user device recognizes another device in its vicinity using a discovery signal, including sending of the discovery signal and receiving of the discovery signal. D2D direct communication refers to direct communication between two user devices, including such processes as call origination, channel measurement, channel feedback, resource scheduling, data transmission, and call termination.
To implement D2D discovery and D2D direct communication between user devices, time synchronization and frequency synchronization need to be ensured between the user devices. In an existing mobile communications system, when communicating with a serving cell, a user device only needs to know and track a synchronization signal of the serving cell. However, in the D2D ProSe, that two or more user devices located in different cells track only synchronization signals corresponding to their respective serving cells can only ensure synchronization between each user device and its corresponding serving cell, but cannot ensure time synchronization and frequency synchronization between the user devices. Consequently, the D2D ProSe between the user devices cannot be ensured.